A soldering apparatus is known in the art as the heat-bonding apparatus in which a heating plate having a heating means is provided in an openable chamber filled with carboxylic acid vapor having a reducing characteristic (see Patent Document 1, for example). Part of the heating plate on which a substrate is placed is flat, and a temperature sensor is provided in the heating plate for sensing the temperature thereof. The heating means is configured to control the temperature of the heating plate based on the sensed temperature of the heating plate.